Improve debug log printing on EMTEST_BROWSER. (#25268)

Author: juj

test/common.py

@@ -2550,7 +2550,7 @@ def browser_open(cls, url):
       elif is_firefox():
         config = FirefoxConfig()
       else:
-        exit_with_error("EMTEST_BROWSER_AUTO_CONFIG only currently works with firefox or chrome.")
+        exit_with_error(f'EMTEST_BROWSER_AUTO_CONFIG only currently works with firefox or chrome. EMTEST_BROWSER was "{EMTEST_BROWSER}"')
       if WINDOWS:
         # Escape directory delimiter backslashes for shlex.split.
         browser_data_dir = browser_data_dir.replace('\\', '\\\\')